1. Field of the Invention
The performance of a crystallization-based separation process is typically limited by the step where the crystalline phase is physically separated from the liquid phase. Since the methods for accomplishing this solid/liquid separation are usually very strongly dependent on the form of the crystals themselves, the approach most often taken to improve this operation is to modify the crystallization process so that it produces suitable crystals. As a result, elaborate and expensive crystallizers are often designed in order to accommodate the crystal size and shape requirements of the solid/liquid separation equipment. Moreover, in the freeze-concentration of food materials, the choice of separation methods is necessarily restricted by special considerations relating to product quality and cost. For example, fruit juices contain volatile flavor components that are retained in the freezing process. The subsequent separation stage should also retain the volatile components in order to preserve the fresh qualities of the freeze-concentrated juice. Because of the high value of food products, product losses are a particularly important consideration in freeze-concentration. This invention relates to an apparatus and process for washing and separating particles from liquid suspensions so as to maximize product recovery and quality and to minimize capital expenditures.
2. Description of the Prior Art
The currently favored separation method for use in freeze-concentration processes is the displacement wash column. In this device, the ice crystals from the freezer are compacted into a porous bed in a cylindrical column. This is done by removing the liquid concentrate through a strainer, leaving behind the amount of liquid that fills the pores of the crystal bed. The crystal bed is then transported through the column by means of a piston or screw device to the far end, where it is melted by a heating element and the resulting liquid is drawn off. Enough back pressure is maintained at this outlet so that some of the melted water is forced back into the column. The ice crystal bed is thus washed as it is displaced from the concentrated liquid portion of the column to the dilute liquid in the other end.
Under normal conditions, this apparatus meets the above-mentioned requirements for maintaining product quality. The recovery is also very high when the ice crystals from the freezer are large enough to make a sufficiently porous bed. In practice, however, food components such as sugars and pectins inhibit suitable ice crystal growth, resulting in a bed having low permeability. If the permeability becomes too low, the wash front between the dilute and concentrated regions of the column becomes unstable and the device ceases to produce any separation at all. Also, when the food material is concentrated to higher levels, not only is the concentration of crystal growth inhibitors higher but the permeability of the bed to the higher viscosity material is diminished. The net effect of these two problems is reduced wash column effectiveness. Multistage wash columns require a crystallizer for each stage, thereby adding considerably to the capital expense.
An alternative to the wash column is the hydrocyclone. Hydrocyclones can be effectively used with particles as small as 10 microns, whereas the wash column typically requires an average particle size of over 300 microns to function.
As with the wash column, the hydrocyclone is closed to the atmosphere during operation, which prevents devolatilization of flavor components. The hydrocyclone also has the advantage that it can be used to separate ice crystals from a concentrate that contains suspended solids (i.e., juice pulp), thereby avoiding a preliminary pulp removal step.
For use in solids washing and decantation, multiple hydrocyclones are used in a countercurrent arrangement. In a countercurrent cascade of conventional, "single inlet" hydrocyclones, the particle slurry and wash fluid are thoroughly mixed before they enter each cyclone. The effluent from the top and bottom outlets from each cyclone are then used to supply the wash for the preceding stage and the slurry for the following stage. The recovery that can be obtained with a properly designed system of this sort is limited only by the number of washing stages that are employed. While recovery is theoretically unlimited, a countercurrent system of conventional hydrocyclones would require an excessive number of stages to meet the stringent recovery specifications of a food concentration process.